Surface acoustic wave (SAW) devices are frequently used in commercial and other applications as RF and IF filters to provide frequency selectivity and other electronic functions. A SAW device depends upon acoustic wave generation. In some cases these acoustic waves are localized to the surface of the substrate. The substrates are piezoelectric materials. Because the acoustic waves in a SAW device often propagate along or very near the surface, SAW devices are generally very sensitive to surface conditions. This sensitivity is not of a chemical or electronic charge nature as it is, for example, in the case of semiconductors, but is of a mechanical nature. For example, foreign material in contact with the surface of a SAW device can change the elastic and inertial properties of the surface thereby altering the attenuation and propagation of acoustic waves moving along on the surface.
Present day SAW devices are typically packaged in hermetic enclosures because of this surface sensitivity. Low cost packaging techniques which are, for example, widely used in connection with semiconductor devices and integrated circuits are not generally used in connection with SAW devices. For example, simple transfer molding of plastic enclosures has not been practicable. In transfer molding, a plastic material in a heated and molten state is forced through channels in a mold to surround the active device die, under pressure, to create a plastic enclosure. This process destroys SAW device performance if it directly coats the surface of the SAW with mold compound.
Nearly all current SAW devices depend upon the use of hermetic packaging techniques for environmental protection because of this problem. Ceramic or metal enclosures with solder or welded seal construction are common. Hermetic packaging is generally costly in terms of both the piece parts required and in the encapsulation processing. As the technology for fabricating SAW die themselves has improved, the packaging costs have become an increasingly larger share of the total manufacturing costs.
Accordingly, there continues to be a need for improved means and methods for encapsulating SAW devices, especially those which permit one to employ low cost plastic encapsulation techniques.
It is an advantage of the present invention, that SAW devices can be encapsulated in plastic using conventional injection or transfer molding techniques in a manner that preserves the desirable electrical characteristics of the encapsulated SAW device.